Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

6.5.2.1 Event indicators
Event indicators are the simplest and least expensive of all power-line disturbance monitors. These indica-
tors collect and display data that are generated by power system variations. They may be dedicated to a sin-
gle type of power system variation or, more typically, they may classify several types of events. Data
generated by the electromagnetic phenomena may be displayed with indicator lights, an illuminated bar
graph, an audible alarm, or some combination of the three. Typically, the time of occurrence of the power
system variation is not recorded by this type of device.
Event indicators collect power system variation data by comparing the steady-state condition of the power
system with one or more threshold parameters. These parameters may be preset or user adjustable. In the
event that the threshold(s) are exceeded, a power system variation is detected and recorded. Comparison of
the steady-state condition and the power system variation is accomplished through the use of analog and/or
digital circuit techniques. These threshold parameters dictate the types and number of power system varia-
tions that are detected by this type of monitor.
Once a power system variation is detected, it may be stored as an amplitude or a total number of occurrences
that exceed the thresholds. Data may be displayed as a numerical value for amplitude or a total count of the
individual power system variations. Some type of illuminated indicator and audible alarms may also be used
to display the data.
6.5.2.2 Text monitors
A second type of power-line disturbance monitor is referred to as a text monitor. As with the event indica-
tors, these devices collect and display power system variations but include several important differences.
Individual power system variations are displayed with an alphanumeric description. Further, these variations
are usually logged as to time of occurrence. The output from this type of monitor may be recorded on paper
tape, electronic media storage, or a combination of the two.

---

IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
33
Generally, text monitors employ the same threshold comparison techniques as those used in the event indica-
tors. The steady-state condition of the power system is compared with preset or user adjustable thresholds.
In the event that one or more of the thresholds is exceeded, measurement data is collected and stored in the
instrument. Electronic circuitry used to perform the comparisons may be based on analog as well as digital
techniques. As with the event indicators, these threshold parameters dictate the number as well as the type of
power system variations that are recorded for future analysis.
The data display techniques offered by the text monitors provide improvements in the quality of data col-
lected for future analysis. When power system variations are detected, a descriptive alphanumeric message is
generated that represents the variations. Accuracy of the data collected and displayed is dependent upon the
measurement parameters and techniques. A further feature of this instrument is that the time of occurrence
of the power system variation is recorded to aid in future data reduction and analysis.